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Special Content

volume-47, 22-28 February 2020

C.V. Raman and National Science Day

Biman Basu

Scientific discoveries are often made out of the blue, as happened, literally, with the Nobel-winning Indian physicist C.V. Raman who postulated the scattering of light by water molecules to be at the root of the deep blue colour of the sea, during a sea voyage. The phenomenon has come to be known as the "Raman Effect". Every year, the 28th of February is celebrated in India as National Science Day to commemorate the day the Raman Effect was discovered in 1928, for which Raman received the Nobel Prize in Physics in 1930. This was the first Nobel Prize for India in the field of Science.

The suggestion to designate the 28th of February as National Science Day was made to the Government in 1986 by the National Council for Science and Technology Communication (NCSTC) of Department of Science & Technology (DST), which was accepted and the government declared the day as National Science Day. NCSTC is the nodal agency to support catalyse and coordinate celebration of the National Science Day throughout the country, particularly in scientific institutions and research laboratories. The first National Science Day was observed on 28 February 1987. Over the years, the National Science Day has been celebrated every year on a specific theme. The theme for 2020 is "Women in Science".

DST has also instituted annual National Awards since 1987 to stimulate, encourage and recognise outstanding efforts in the area of science popularisation and communication and in promoting scientific temper, which are given away on National Science Day every year.

There is a long story behind the National Science Day - the discovery of Raman Effect. It all started during a sea voyage from England to India in 1921, when Raman noticed the blue opalescence of the Mediterranean Sea and wondered about the origin of this beautiful colour. Raman was aware of the British physicist Lord Rayleigh's explanation that the colour of the sea was due to the reflection of the blue sky, but he did not accept it. As was his wont, Raman immediately confirmed his belief by using a polarising Nicol quartz prism that he carried in his pocket. This made him confident that the colour of the sea was indeed due to something else, probably scattering of light by the water molecules, and not reflection of the sky. It was sheer innovative thinking that made Raman challenge an accepted theory. Moreover, not only did he postulate a new explanation of the sea colour but he himself designed the experiment and built the instruments that he used to test it.

On his return to Kolkata (formerly Calcutta) in September 1921, Raman started experimenting with scattering of light by different liquids at the Indian Association for the Cultivation of Science that ultimately led to the discovery of the Raman Effect. When he started the scattering experiments Raman had a hunch that the study of light-scattering might lead to the deepest problems of physics and chemistry and it was this belief that led him focus on this subject in his research in Kolkata.

Although known as Raman Effect, the phenomenon was discovered jointly by Raman and his colleague physicist K.S. Krishnan. Together they started working on scattering of light by different liquids and gases. At first, Raman and Krishnan could not understand what was happening, but it soon became clear that the phenomenon could be the optical analogue of the Compton Effect, also known as Compton scattering,which was quite well known at that time.Compton Effect is the scattering of X-rays by electrons; Raman thought that here too, the wavelength of the scattered light was changing due to inelastic scattering.

Scattering of light by molecules and particles is a common phenomenon. When light is scattered by fine dust particles, for example, which makes a beam of light visible in a darkened room, it is known as 'Tyndall effect'. When light is scattered by molecules smaller than the wavelength of light, such as molecules of gases in air, it is known as 'Rayleigh scattering', which gives the daytime sky its blue colour due to scattering of the blue wavelengths out of the white colour of sunlight. Both Tyndall effect and Rayleigh scattering are examples of 'elastic scattering', meaning that no energy is exchanged between the photon of light and the scattering particle and as a result, there is no change in the wavelength of the scattered light.

But what Raman and his colleagues observed in their scattering experiments with transparent liquids was different. Here, apart from scattered light of the same wavelength as the incident beam, fainter secondary radiation of wavelengths slightly lower and higher wavelengths was also observed. The change in wavelength in this case was due to inelastic scattering in which exchange of energy took place between the incident light beam and the scattering molecules. 

Although the scattering of light observed by Raman and Krishnan appears similar to the scattering of X-rays in Compton Effect, the similarity ends here. Compared to the intensity of scattered X-rays observed in Compton scattering, the intensity of scattered light in Raman's case was extremely feeble, as only one in a million of the scattered light particles, or photons, actually exhibits the change in wavelength,which made its observation extremely difficult.

Raman's genius lay in his clever use of a set of complementary light-filters in the path of the incident and scattered beams to isolate the scattered beams with changed wavelengths. To produce an incident beam of sunlight of great intensity required for the experiment, he used a 7-inch refracting telescope in combination with a short-focus lens to create an extremely bright beam of sunlight.

Finally, on 28 February 1928, they discovered something strange, as recorded in his diary by Krishnan

February 28, Tuesday

Went to the Association only in the afternoon. Prof. [Raman] was there and there we proceeded to examine the influence of the wavelength of the incident light on the phenomenon. Used the usual blue-violet filter coupled with a [green coloured] uranium glass, the range of wavelengths transmitted by the combination being much narrower than that transmitted by the blue-violet filter alone. On examining the track with a direct vision spectroscope, we found to our great surprise [that] the modified scattering was separated from the scattering corresponding to the incident light by a dark region.

Raman realised that this was a totally different phenomenon not known earlier. Using his innovative method Raman was thus able to study light scattering by a large number of transparent liquids and also compressed gases such as carbon monoxide and nitrous oxide, and crystalline ice and optical glasses, all of which were found to show change in wavelength in the scattered light. The change in wavelength of the scattered light was found to depend on the nature of the scattering molecule. This was the Raman Effect. A newspaper announcement was made the following day, 29 February (being a leap year). The first scientific paper on the new phenomenon was published in the journal Nature on 31 March 1928.The significance of Raman's work can be judged from the award of the Nobel Prize in 1930, within two years of the discovery.

Raman had realised that the scattering of light offered an alternative to X-ray diffraction as a means of identifying compounds. But not until the advent of more powerful, less expensive lasers in the 1970s and '80s and advances in digital imaging in the 1990s, did scientists begin researching applications for Raman spectroscopy.Today, Raman spectroscopy is an inseparable part of scientific research, especially for the analysis of a wide range of materials and systems.

Thus, as we have seen, the discovery of Raman Effect was the result of a combination of curiosity, astute observation and innovative thinking that led Raman to design his own experiment and hit upon an entirely new scientific phenomenon. In the same spirit, the observance of the National Science Day is aimed at encouraging curiosity, out-of-the-box thinking and motivating people to take a rational approach to problem solving. Despite many significant achievements in science, certain sections of our society are still guided by blind faith and beliefs, which is reflected in the quality of decision making on developmental issues and is often detrimental to healthy development of the nation. This needs to change.

The basic objective of the observance of National Science Day is to spread the message of the importance of science and its application among the people. The entire nation celebrates the National Science Day as a science festival by organising workshops, talks, showing of science movies, exhibitions based on themes and concepts, live projects, debates, quiz competitions, seminars, etc. Students of schools and colleges demonstrate various science projects. The major scientific institutions of the country observe "Open Day" on the occasion to allow common citizens, especially students, to visit and see how scientists work. The aim is to inculcate scientific temper in the minds of people of all ages,which is essential to accelerate the pace of development of the country to take it forward.

(The author is a science communicator and consultant. e-mail: bimanbasu@gmail. com)

Views expressed are personal.

Image Courtesy : Google